If you are serving the file from code, add the Content-Disposition header:
response.setHeader("Content-Disposition", "attachment;filename=myFile.txt");
If you are serving the file from the filesystem, add the download attribute to the a element:
<a download="myFile.txt" href=...
If you are serving the file from memory, use a data: url:
<a download="myFile.txt" href="data:application/octet-stream;base64,...